Oxidative Addition of Carbon−Hydrogen Bonds

Published on Web 06/18/2003

Reductive Elimination/Oxidative Addition of Carbon-Hydrogen Bonds at Pt(IV)/Pt(II) Centers: Mechanistic Studies of the Solution Thermolyses of TpMe2Pt(CH3)2H Michael P. Jensen,‡ Douglas D. Wick,‡ Stefan Reinartz,§ Peter S. White,§ Joseph L. Templeton,*,§ and Karen I. Goldberg*,‡ Contribution from the Department of Chemistry, Box 351700, UniVersity of Washington, Seattle, Washington 98195-1700, and the Department of Chemistry, UniVersity of North Carolina, Chapel Hill, North Carolina 27599-3290 Received September 9, 2002; E-mail: [email protected], [email protected]

Abstract: Reductive elimination of methane occurs upon solution thermolysis of κ3-TpMe2PtIV(CH3)2H (1, TpMe2 ) hydridotris(3,5-dimethylpyrazolyl)borate). The platinum product of this reaction is determined by the solvent. C-D bond activation occurs after methane elimination in benzene-d6, to yield κ3-TpMe2PtIV(CH3)(C6D5)D (2-d6), which undergoes a second reductive elimination/oxidative addition reaction to yield isotopically labeled methane and κ3-TpMe2PtIV(C6D5)2D (3-d11). In contrast, κ2-TpMe2PtII(CH3)(NCCD3) (4) was obtained in the presence of acetonitrile-d3, after elimination of methane from 1. Reductive elimination of methane from these Pt(IV) complexes follows first-order kinetics, and the observed reaction rates are nearly independent of solvent. Virtually identical activation parameters (∆Hqobs ) 35.0 ( 1.1 kcal/mol, ∆Sqobs ) 13 ( 3 eu) were measured for the reductive elimination of methane from 1 in both benzene-d6 and toluene-d8. A lower energy process (∆Hqscr ) 26 ( 1 kcal/mol, ∆Sqscr ) 1 ( 4 eu) scrambles hydrogen atoms of 1 between the methyl and hydride positions, as confirmed by monitoring the equilibration of κ3TpMe2PtIV(CH3)2D (1-d1) with its scrambled isotopomer, κ3-TpMe2PtIV(CH3)(CH2D)H (1-d1′). The σ-methane complex κ2-TpMe2PtII(CH3)(CH4) is proposed as a common intermediate in both the scrambling and reductive elimination processes. Kinetic results are consistent with rate-determining dissociative loss of methane from this intermediate to produce the coordinatively unsaturated intermediate [TpMe2PtII(CH3)], which reacts rapidly with solvent. The difference in activation enthalpies for the H/D scrambling and C-H reductive elimination provides a lower limit for the binding enthalpy of methane to [TpMe2PtII(CH3)] of 9 ( 2 kcal/mol.

Introduction

Exploitation of the vast reserves of alkanes as chemical feedstocks will require the development of methodology for the selective functionalization of saturated hydrocarbon C-H bonds. Alkane C-H bonds are difficult to activate selectively, a result of their instrincally high bond strength and low acidity, combined with a strong thermodynamic proclivity toward overoxidation. Despite intensive investigations of several natural and artificial systems that activate such bonds, the development of a significant industrial process remains elusive.1-3 One promising approach to solving this problem takes advantage of the selectivity of C-H activation reactions of low-valent, low‡ §

University of Washington. University of North Carolina.

(1) Recent reviews: (a) Shilov, A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2879. (b) Jones, W. D. Top. Organomet. Chem. 1999, 3, 9. (c) Sen, A. Top. Organomet. Chem. 1999, 3, 80. (d) Crabtree, R. H. J. Chem. Soc., Dalton Trans. 2001, 2437. (e) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507. (2) (a) Shilov, A. E. ActiVation of Saturated Hydrocarbons by Transition Metal Complexes; D. Riedel: Dordrecht, The Netherlands, 1984. (b) Shilov, A. E.; Shul’pin, G. B. ActiVation and Catalytic Reactions of Saturated Hydrocarbons in the Presence of Metal Complexes; Kluwer: Boston, 2000. (3) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Acc. Chem. Res. 1995, 28, 154. 8614

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coordinate complexes of the late transition metals. Thirty years ago, Shilov reported the selective oxidation of methane to methanol and methyl chloride by Pt(II) and Pt(IV) chloride salts in aqueous solution.2 Although a Pt(II) species is the catalyst in this reaction, consumption of Pt(IV) as a stoichiometric oxidant makes the system impractical. More recently, Periana et al. used a discrete (bipyrimidine)Pt(II) complex in sulfuric acid with SO3 as the oxidant to convert methane to methyl bisulfate.4 Further advances have shown the potential of H2O2 and O2 as oxidants in platinum(II)-catalyzed alkane oxidation reactions.5 The first step in platinum(II)-catalyzed alkane oxidation reactions is the activation of a C-H bond, resulting in formation of a Pt(II) alkyl. This occurs either via oxidative addition followed by deprotonation of the resulting Pt(IV) alkyl hydride or by direct deprotonation of a Pt(II) σ-alkane complex.6 Evidence obtained primarily from protonation studies of model (4) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H. Science 1998, 280, 560. (5) (a) Thorn, D. L.; Roe, D. C.; DeVries, N. J. Mol. Catal. A 2002, 189, 17. (b) Lin, M.; Shen, C.; Garcia-Zayas, E. A.; Sen, A. J. Am. Chem. Soc. 2001, 123, 1000. (6) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed. 1998, 37, 2180, and references therein. 10.1021/ja028477t CCC: $25.00 © 2003 American Chemical Society

Solution Thermolyses of TpMe2Pt(CH3)2H Scheme 1

Pt(II) alkyl complexes supports the oxidative addition pathway.6,7 A number of C-H activation reactions at Pt(II) characterized by alkyl/aryl exchange with hydrocarbon solvents have been documented, but since the proposed bis(hydrocarbyl) Pt(IV) hydride oxidative addition intermediates were not observed, the involvement of Pt(IV) could not be unambiguously established.8-10 A common feature of Pt(II) complexes that activate C-H bonds is the presence of a good leaving group (triflate, perfluoropyridine, water, trifluoroethanol, etc.) in the squareplanar Pt(II) coordination sphere. Replacement of this ligand (L) with a hydrocarbon forms the Pt(II) σ-alkane complex, thought to be the first intermediate in the C-H activation reaction (Scheme 1).6 While convincing evidence has been presented for an associative displacement of the ligand L by the alkane in one type of ligand system,8b,11 the mechanism of this exchange reaction likely depends on the specific ligand set and charge on the metal center (see Discussion section). Facile C-H bond cleavage occurs from the σ-alkane intermediate via either σ-bond metathesis or oxidative addition. The oxidative addition pathway is shown in Scheme 1 and would produce a five-coordinate bis(hydrocarbyl) hydride Pt(IV) intermediate. Reversal of the reaction sequence but involving the original hydrocarbyl ligand completes the exchange. Detailed mechanistic studies of these C-H bond activation reactions have been hampered by the elusive nature of the five-coordinate Pt(IV) alkyl hydride species. Five-coordinate Pt(IV) alkyls have been consistently proposed as intermediates in oxidative addition/ reductive elimination reactions at Pt(II)/Pt(IV) involving C-H, C-C, and C-X bonds,12-17 and recently, the first examples of five-coordinate Pt(IV) complexes, namely, the trimethyl com(7) Wik, B. J.; Lersch, M.; Tilset, M. J. Am. Chem. Soc. 2002, 124, 12116. (8) (a) Zhong, H. A.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2002, 124, 1378. (b) Johansson, L.; Ryan, O. B.; Rømming, C.; Tilset, M. J. Am. Chem. Soc. 2001, 123, 6579. (c) Johansson, L.; Tilset, M.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2000, 122, 10846. (d) Heiberg, H.; Johansson, L.; Gropen, O.; Ryan, O. B.; Swang, O.; Tilset, M. J. Am. Chem. Soc. 2000, 122, 10831. (e) Johansson, L.; Ryan, O. B.; Tilset, M. J. Am. Chem. Soc. 1999, 121, 1974. (9) (a) Holtcamp, M. W.; Henling, L. M.; Day, M. W.; Labinger, J. A.; Bercaw, J. E. Inorg. Chim. Acta 1998, 270, 467. (b) Holtcamp, M. W.; Labinger J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1997, 119, 848. (10) (a) Brainard, R. L.; Nutt, W. R.; Lee, T. R.; Whitesides, G. M. Organometallics 1988, 7, 2379. (b) Peters, R. G.; White, S.; Roddick, D. M. Organometallics 1998, 17, 4493. (11) (a) Johansson, L.; Tilset, M. J. Am. Chem. Soc. 2001, 123, 739. (b) Procelewska, J.; Zahl, A.; van Eldik, R.; Zhong, H. A.; Labinger, J. A.; Bercaw, J. E. Inorg. Chem. 2002, 41, 2808. (12) (a) Hill, G. S.; Rendina, L. M.; Puddephatt, R. J. Organometallics 1995, 14, 4966. (b) Jenkins, H. A.; Yap, G. P. A.; Puddephatt, R. J. Organometallics 1997, 16, 1946. (c) Fekl, U.; Zahl, A.; van Eldik, R. Organometallics 1999, 18, 4156. (d) Prokopchuk, E. M.; Puddephatt, R. J. Organometallics 2003, 22, 563. (e) Prokopchuk, E. M.; Puddephatt, R. J. Organometallics 2003, 22, 787. (13) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996, 118, 5961.

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plex (nacnac)PtIVMe3 (nacnac- ) [{(o-iPr2C6H3)NC(CH3)}2CH]-)18 and the silyl dihydride complex [κ2-((HpzMe2)BH(pzMe2)2)Pt(H)2(SiEt3)][BAr′4] (BAr′4 ) B(C6H3(CF3)2)4),19 have been isolated and structurally characterized.20 Structurally analogous five-coordinate Pt(IV) alkyl hydrides are expected to undergo facile reductive elimination. The first examples of C-H oxidative addition reactions to Pt(II) to yield stable six-coordinate Pt(IV) alkyl hydrides were reported in 1997 (Scheme 2).21 Reaction of K[κ2-TpMe2PtII(CH3)2] (TpMe2 ) hydridotris(3,5-dimethylpyrazolyl)borate) with B(C6F5)3 in hydrocarbon solvent (R-H) produces the Pt(IV) di(hydrocarbyl) hydride species κ3-TpMe2PtIV(CH3)(R)(H) (R ) phenyl, cyclohexyl, pentyl). Abstraction of a methide group from the starting Pt(II) anion was proposed to generate the key threecoordinate species [κ2-TpMe2PtII(CH3)], which reacts with hydrocarbons. Oxidative addition of the C-H bond produces the five-coordinate Pt(IV) hydrocarbyl hydride, which is rapidly trapped by coordination of the third arm of the TpMe2 ligand to yield a stable six-coordinate Pt(IV) complex (Scheme 2). A similar proposal of C-H bond activation by a four-coordinate κ2-TpMe2 Rh(I) σ-alkane species followed by attachment of the third pyrazolyl to the Rh(III) alkyl hydride product was supported by ultrafast time-resolved infrared experiments.22 The surprising stability of the Pt(IV) dialkyl hydrides κ3TpMe2PtIV(CH3)(R)(H) has been attributed to the chelating ability of the tridentate ligand, which inhibits formation of a fivecoordinate intermediate from which reductive elimination is expected to take place.23 Other examples of unusually stable Pt(IV) alkyl hydrides also contain ligands that do not easily dissociate from the metal center.12b-d,17,24 Direct examination of the mechanism of C-H bond activation that occurs upon reaction of K[κ2-TpMe2PtIIMe2] with B(C6F5)3 in hydrocarbon solvent was constrained by the requisite bimolecular activation step with B(C6F5)3 and the presence of competing reactions. We now report that this C-H activation can be successfully studied via investigation of the microscopic reverse reaction, C-H reductive elimination from the known compound κ3-TpMe2PtIV(CH3)2(H) (1).23 At elevated temperature, C-H reductive elimination from 1 cleanly generates CH4 and a Pt(II) species that can activate hydrocarbon C-H bonds. This (14) (a) Goldberg, K. I.; Yan, J. Y.; Breitung, E. M. J. Am. Chem. Soc. 1995, 117, 6889. (b) Williams, B. S.; Holland, A. W.; Goldberg, K. I. J. Am. Chem. Soc. 1999, 121, 252. (c) Crumpton, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2000, 122, 962. (d) Williams, B. S.; Goldberg, K. I. J. Am. Chem. Soc. 2001, 123, 2576. (15) (a) Brown, M. P.; Puddephatt, R. J.; Upton, C. E. E. J. Chem. Soc., Dalton Trans. 1974, 2457. (b) Roy, S.; Puddephatt, R. J.; Scott, J. D. J. Chem. Soc., Dalton Trans. 1989, 2121. (c) Hill, G. S.; Yap, G. P. A.; Puddephatt, R. J. Organometallics 1999, 18, 1408. (16) Fekl, U.; Goldberg, K. I. AdV. Inorg. Chem. 2003, 54, in press. (17) Puddephatt, R. J. Coord. Chem. ReV. 2001, 219, 157. (18) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2001, 123, 6423. (19) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. J. Am. Chem. Soc. 2001, 123, 6425. (20) Puddephatt, R. J. Angew Chem., Int. Ed. 2002, 41, 261. (21) Wick, D. D.; Goldberg, K. I. J. Am. Chem. Soc. 1997, 119, 10235. (22) Bromberg, S. E.; Yang, H.; Asplund, M. C.; Lian, T.; McNamara, B. K.; Kotz, K. T.; Yeston, J. S.; Wilkens, M.; Frei, H.; Bergman, R. G.; Harris, C. B. Science 1997, 278, 260. (23) O’Reilly, S. A.; White, P. S.; Templeton, J. L. J. Am. Chem. Soc. 1996, 118, 5684. (24) (a) Canty, A. J.; Dedieu, A.; Jin, H.; Milet, A.; Richmond, M. K. Organometallics 1996, 15, 2845. (b) Hill, G. S.; Vittal, J. J.; Puddephatt, R. J. Organometallics 1997, 16, 1209. (c) Prokopchuk, E. M.; Jenkins, H. A.; Puddephatt, R. J. Organometallics 1999, 18, 2861. (d) Haskel, A.; Keinan, E. Organometallics 1999, 18, 4677. (e) Vedernikov, A. N.; Caulton, K. G. Angew. Chem. Int Ed. 2002, 41, 4102. (f) Vedernikov, A. N.; Caulton, K. G. Chem. Commun. 2003, 358. J. AM. CHEM. SOC.

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Jensen et al.

Scheme 2

Scheme 3

Figure 1. Distribution of species vs time for the thermolysis of 1 in C6D6 at 110 °C (b ) 1, 9 ) 2-d6, 2 ) 3-d11).

approach has accommodated detailed kinetic and mechanistic studies of the C-H bond cleavage/formation reaction. Our results are best explained by the involvement of a Pt(II) σ-alkane intermediate and a dissociative mechanism for the loss of alkane from this intermediate. The unsaturated [κ2-TpMe2PtII(CH3)] fragment is strongly implicated in these C-H activation reactions. A minimum binding enthalpy of methane to the Pt(II) species [TpMe2PtII(CH3)] was also calculated. Results

Thermolysis of TpMe2PtIV(CH3)2H (1) in C6D6. The Pt(IV) complex TpMe2PtIV(CH3)2H (1)23 was heated in C6D6 at 110 °C, and the thermal reaction was monitored periodically at room temperature by 1H NMR spectroscopy. Reductive elimination of methane (δ 0.15) was observed, and the reaction followed first-order kinetic behavior with respect to the disappearance of 1 (kobs(1) ) 6.73 × 10-5 s-1, Table S1). The Pt(IV) methyl phenyl deuteride complex TpMe2PtIV(CH3)(C6D5)(D) (2-d6)21 was generated by activation of the C6D6 solvent (Scheme 3). The initial Pt(IV) product 2-d6 underwent further reaction under the thermolysis conditions to eliminate methane-d1 (with some methane-d2, see below) and form the Pt(IV) diphenyl deuteride 8616 J. AM. CHEM. SOC.

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complex TpMe2PtIV(C6D5)2(D)23 (3-d11) (Scheme 3). The course of this reaction sequencesthe disappearance of 1, the appearance and disappearance of 2-d6, and the appearance of 3-d11swas easily monitored via the TpMe2 methine region in the 1H NMR spectrum. Monitoring the relative intensities of each set of methine signals against the combined total over time yielded kinetic data that were fit to the known solution of differential equations for consecutive, irreversible first-order reactions,25 Figure 1. Despite the apparent simplicity of this chemistry, several complications warrant comment. First, the kinetic fits gave a small but reproducible systematic deviation for the concentrations of 2-d6 and 3-d11. Note that the fit as shown in Figure 1 slightly overestimates the accumulation of the final product 3-d11 at early reaction times and then slightly underestimates the concentration of 3-d11 toward the end. Similarly, the parameters underestimate the concentration of 2-d6 at early reaction times and overestimate it at later times. Such errors would result if rate constant kobs(2) (Scheme 3) increased with time. In addition, monitoring evolved methane isotopomers by 1H NMR indicated that CH4 accumulated immediately, followed by CH3D after the expected induction period (Scheme 3). However, CH2D2 also appeared after a longer induction. Furthermore, isotopomers of 2-d6 and 3-d10 with protons in the hydride position as well (25) Espenson, J. H. Chemical Kinetics and Reaction Mechanisms, 2nd ed.; McGraw-Hill: New York, 1995; p 71.

Solution Thermolyses of TpMe2Pt(CH3)2H

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Scheme 4

Figure 2. Distribution of species vs time for the thermolysis of 1-d7 in C6D6 at 110 °C (b ) 1-d7, 9 ) 2-d9, 2 ) 3- d11).

as the phenyl ring positions were observed by 1H NMR spectroscopy. These observations are consistent with a relatively fast exchange of hydrogen atoms between the hydride and methyl positions, coupled with a slow exchange of hydride and phenyl positions in the intermediate species 2-d6 (Scheme 4). In this way, another deuteron can leak into the second reductive elimination of methane (second reaction in Scheme 3), producing CH2D2 and perturbing the observed rate constant through a weighted sum of kH and kD, which continuously increases in magnitude. Fortunately, the kinetic effect is small (